This application relates to highly absorbent fibers, for example, viscose rayon, hydroxypropylcellulose, and hydroxyethylcellulose, made from wood pulp or other cellulosic materials, which are useful in the production of nonwoven articles such as diapers, tampons, sanitary napkins, medical sponges, soil mulches, wiping cloths, and the like. Each of these applications requires a material having a high capacity for absorbing and retaining water and other aqueous fluids, particularly, body fluids. Cellulosic fibers have found wide use in these and similar applications because of the hydrophilic nature of the cellulose molecule and its fibrous structure which contributes integrity, form, shape, wicking ability, and liquid retention to a nonwoven material.
Some examples of attempts to increase the absorbency of rayon fibers can be found in U.S. application Ser. No. 330,378, filed Feb. 7, 1973, and U.S. Pat. No. 3,844,287 where alkali metal and ammonium salts of polyacrylic acid are incorporated in regenerated cellulose to increase the fluid absorbency of rayon fibers produced therefrom. Other examples of hydrophilic polymers incorporated into viscose to increase the hydrophilic properties of the fiber are the addition of carboxymethylcellulose and carboxyethyl starch as described in U.S. Pat. Nos. 3,423,167 and 3,847,636, respectively.
It has been previously known that the water absorption of a material depended upon the degree of hydrophilicity it exhibited. Such determinations of water absorption as swelling and water retention under centrifugation can be considered to be dependent on the degree of ionization of the material. Hence, the more strongly acidic that a material incorporated in a fiber is, the more absorptive it will be. In the case of polyacrylic acid, since it is more acidic, i.e., more highly ionized, than polymethacrylic acid, it would be expected that rayon fibers containing injected polyacrylic acid would have a higher water retention value than fibers containing injected polymethacrylic acid. This has been demonstrated, as will be shown later.
One problem that has been found with the approach of incorporating the anionic hydrophilic polymers into cellulosic fibers is the adverse effect on carding properties. An important property which fibers must have to be used in the applications previously mentioned, and especially in the preparation of tampons, is the capability of being carded on conventional carding equipment. The incorporation of polyacrylic acid into viscose rayon fibers by the injection method was found to decrease the sliver cohesiveness significantly. The cohesiveness property is very important in the commercial processing of rayon fibers into useful products such as tampons. If the cohesiveness is too low, then the fibrous material will not cohere, necessitating a reduction in processing speed or other process modifications, or even making the fiber impossible to process. Therefore, it is a primary object of this invention to provide a rayon fiber having improved fluid absorption without an excessive loss in fiber cohesiveness and consequent difficulty in carding the fibers for processing into useful articles.